The initial installation tension is the most critical factor to a successful synchronous belt drive. A synchronous belt drive is used to drive machines where a timed or synchronous condition is required. A synchronous belt uses a toothed belt to achieve the desired timing effect.
The conventional synchronous belt drive requires the tension ratio, which is defined as the ratio between the tight side tension and slack side tension, to be about eight (8) for good results. Using a tension ratio of about 8 proves to be a good approach for the constant load drive, for example, the fan drive, conveyer belt drive, and so on.
With high quality synchronous belt development, automotive industries explored the possibility of using synchronous belts to replace the metal chain for the transfer case applications. The traditional transfer case uses a metal chain to split the power from the engine to drive the front wheel and rear wheels. This arrangement requires a chain guide, sealed case, and oil lubrication. The advantage of the belt drive is the complete elimination of the oil, for a so-called dry transfer case.
The challenge of a dry transfer case is the belt initial installation tension. Because the large variation of the torque passing through the transfer case, if a tension ratio of 8 is used the required initial tension is significantly high. The high initial tension results in a poor drive efficiency and belt noise. On the other hand, if the initial tension is too low, the belt may jump a tooth when high torque is encountered.
The concept of using belt backward buckling to achieve the zero tension drive has been explored in the prior art. The idea is to use a belt pitch length longer than the drive length, and let the belt buckle backward on the slack side span. In the prior art device two bearings are placed at the exit and entrance of the slack span and serve as the guide to lead the belt into the desired backward buckling. When the forward torque is applied, the extra belt length is taken up by the backward buckling that forms an arc shape. The belt back bending stiffness is relevant to allowing the belt buckling in the backward direction.
In comparison to a metal chain drive, there is no bending stiffness due to the flexible chain link connections, therefore making it impossible to achieve the backward buckling.
Representative of the art is U.S. Pat. No. 8,308,589 which discloses a belt and chain drive for vehicles or for use in drive technology with an input shaft and an output shaft supported on a frame, the input shaft and the output shaft project out of the frame, with the following features: a) gear transmissions with gear wheels, which are embodied as belt and chain drives, are located between the input shaft and the output shaft, b) all of the gear wheels are constantly in rotation during operation, c) the gear transmissions located between the input shaft and the output shaft are embodied as belt and chain drives with toothed belts as traction mechanisms and with pulleys as gear wheels, d) the traction mechanism are reinforced with aramid, Kevlar, carbon fibers or other fibrous materials, is characterized in that e) the traction mechanism is pressed into a kidney-like shape onto the pulleys by at least one component during no-load rotation, and that under the effect of load this component does not touch the traction mechanism, and f) the kidney-like shape of the traction mechanism during no-load rotation is formed by a convex curvature of the driving side and by a concave curvature of the slack side, and g) the kidney-like shape of the traction mechanisms under load is formed by a straight shape of the driving side and by an intensified concave curvature of the slack side.
What is needed is a toothed belt drive having a drive length which is less than a toothed belt length such that the toothed belt forms a free-standing arcuate span between the first sprocket and the second sprocket on a toothed belt compression span. The present invention meets this need.